This invention relates generally to an inductive coil for use in electrical circuits, and more specifically relates to a tunable coil wound around a core, and still more specifically relates to a tunable coil assembly including turns of the coil wound around a toroid core and a variable cylindrical core.
Electrical filters are used to pass or reject a band of frequencies, or may be designed to pass frequencies below a designated frequency ("low pass") or pass frequencies above a designated frequency ("high pass"). Various electrical networks may be used to provide the desired frequency response, such as for example, the simple L-C (inductance-capacitance) section. A plurality of L-C sections may be cascaded together to increase selectivity and sharpness of the response curve.
The inductors for the filters may be air coils, either self supported or coils wound on electrically insulated coil forms (such as ceramic or glass). Iron-core coils or ferrite-core coils may also be used. The iron-cores are generally powdered iron or laminated iron or steel. If the coils are wound on a ring shape instead of a cylindrical form, they are referred to as toroids. Toroids are characterized by their relatively high Q, and special shape of the generated magnetic field.
In order to adjust the inductance or peak or set the response curve of the tuned circuit, variable coils or variable capacitors are used. The variable coils are generally wound on a hollow cylindrical form with a ferrite, powdered iron or brass core movable inside the cylindrical form. The core may be threaded and incrementally movable along internal threads formed inside the cylindrical form. The variable coil may constitute the entire inductive part of a tuned circuit, but usually is in series with a fixed inductor.
When designing a sharp response or narrow band electrical filter, the Q or quality factor of the circuit is a key consideration. Q may be defined as the ratio of the resonant frequency to the bandwidth of the frequency response curve at the 3 db points, or Q equals (fo/f1 minus f2). Therefore, as the bandwidth of the filter is increased, the Q is decreased, but as the bandwidth is narrowed the Q is increased.
The Q or quality factor of the coil is commonly defined as the ratio of the reactance of the coil and its DC resistance, and is therefore, dependent upon frequency. When using a variable coil, alone or with a fixed coil, the Q usually varies over the range of adjustment of the variable coil.
In many instances, when a sharp or narrow band response is required, the Q must be maintained within tight and exact tolerances. Thus, to prevent any degradation of the Q when setting the resonant frequency, particularly in the MegaHertz range, it is a common practice to cut or file the capacitor in the capacitance arm of the tuned circuit and thereby varying capacitance instead of inductance. This technique has been effective in maintaining constant Q, as the tuned circuit is set at the precise design center frequency. However, the manufacture of these tuned circuits is a problem, since it requires substantial skill on the part of the worker, and frequently too much of the capacitor is cut or filed away, which, of course, cannot be retrieved. Consequently, the damaged capacitor must be replaced, and the filing of the capacitor started over again for tuning the circuit.
Generally, toroids are used in the inductive arm of very high Q circuits. To vary the inductance, a variable coil is often connected in series with the toroid coil. The variable coil is physically placed in the circuit spaced from the toroid. However, upon adjustment of the variable coil, the Q of the tuned circuit usually varies appreciably over the adjustment range of the variable coil. Thus, due to the variation in Q upon adjusting the inductance in the circuit, the advantage of using a toroid coil was, heretofore, substantially reduced. When it was critical to utilize the high Q characteristic of the toroid coil, the variable coil was not used, but instead the capacitor was trimmed for tuning the circuit, as aforedescribed.